1. Field of the Invention
This invention relates to a simple device for accurate and repeatable measurements of optical wavelengths, especially Bragg wavelengths for fiberoptic Bragg gratings and wavelengths, as well as laser effect and signal/noise ratios for each laser in a wavelength multiplexed communication system.
2. Description of the Related Art
A fiberoptic Bragg grating (FBG) is a permanent, photo-induced periodic modulation of the refractive index in the core of an optical fiber which reflect light within a narrow wavelength area centred around the Bragg wavelength, It is known that an FBG, though measuring of the Bragg wavelength may be used to detect an arbitrary perturbation, such as temperature or strain (by the location of the fiber grating), which changes the physical period of the modulation in the refractive index and/or the effective refractive index as seen by the light propagating along the fiber grating, and thus the Bragg wavelength. An important property of the FBG sensors is that the reflected Bragg wavelength from a Bragg sensor element, with precise calibration, is an absolute measure of the amplitude of the perturbation(s) in the fiber grating. In sensor uses in which for example temperature or strain is measured, the Bragg wavelength, being in the range of 1 xcexcm, must be measured with a resolution, accuracy and repeatability of approx. 1 pm, which is only 0.3-1% of the reflection bandwidth of the fiber grating. With a typical Bragg wavelength of 1.55 xcexcm a shift in the Bragg wavelength being 1 pm will correspond to a change in stretch or temperature of 1 xcexcm or 0.1xc2x0 C., respectively.
Another important property of the FBG sensor is that many FBG sensors may be multiplexed along one or more optic fibres by writing the fiber gratings with different Bragg wavelengths which does not overlap during sensor operation, to obtain quasi-distributed measurements with FBG sensor elements position at arbitrary positions at distances from a few millimeters to tens of kilometers.
U.S. Pat. No. 4,994,419 shows an example of time multiplexing by using one in a number of sensor gratings along the same fiber as a method for having a large number of gratings along one fiber being independent of wavelength area being allocated to each sensor. The distance between each sensor should be relatively large, large enough to separate the reflected pulses in time (typically  greater than 10 m, corresponding to 0.1 ps delay). The reflected pulses from each grating is in the figure reflected from an analysis grating which may be biased to overlap each sensor grating within the time window of this sensor grating. The wavelengths of the sensor gratings are approximately equal, but it is advantageous if they are slightly different so that multiple reflection and thus cross talk is reduced. The grating wavelengths may overlap, which makes the system unsuitable for use in a wavelength multiplexed system or for measuring optical wavelengths.
It is known that one or more FBG sensor wavelengths may be measured using a broadband source, e.g. a light emitting diode (LED) or a super luminescent fiber source (SFK) in combination with an adjustable optical filter, for example an adjustable Fabry-Perot (F-P) filter controlled by a piezoelectric transducer (PZT) [Kersey, A. D., Berkoff, T. A., and Morey, W. W., xe2x80x9cMultiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter,xe2x80x9d Optics Letters, Vol. 18, s. 1370-1372, 1993), or, as an alternative, an adjustable laser source, for example an external cavity semiconductor laser with an external adjustable FBG reflector [U.S. Pat. No. 5,401,956], if the spectrum of the laser covers all the possible FBG sensor wavelengths. These techniques makes simultaneous wavelength, demultiplexing and demodulation (exact determination of wavelength) possible for several different FBG sensors. To obtain accurate, repeatable wavelength measurements with these techniques a reference system may be used based on a fixed Fabry-Perot filter and a reference filter with separate detector channels [Norwegian patent application 1997.0674). A disadvantage with such a system is the relatively large component and production costs.
A simpler and less expensive system for reading the wavelength of an FBG filter is based on a broadband source and an optical flank filter [U.S. Pat. No. 5,319,435], where the reflected light from an FBG is split through a fiberoptic coupler and where a part of the light is sent through a flank filter, a shift in the Bragg wavelength thus resulting in a change in the transmitted power, and to a detector, while another part is sent directly to another detector. The ratio between the detector signals is a unambiguous measure of the Bragg wavelength. Such a system has a fast time response, but is not suitable for wavelength multiplexing of several sensors along the same fiber, will usually be polarized and is sensitive to temperature changes in the flank filter. A related technique is based on the use of two adjustable FBGs with partial spectral overlapping as receiving filter where the Bragg wavelength in one FBG sensor will be on the flank of the two receiving grating each reflecting light to one detector, so that the ratio between the detector signals will be a measure of the Bragg Wavelength [U.S. Pat. No. 5,410,404]. This technique provides good time response and uses inexpensive components, but has the disadvantage that it, in addition to two gratings requires three couplers and two detectors for each sensor channel.
Use of several couplers gives optical loss and thus reduced signal. For four wavelength multiplexed sensors along one fiber 15 couplers and 8 detectors are required, in addition to 8 receiver gratings.
Another FBG sensor readout technique being based on the use of adjustable gratings is described in UK patent application GB 2268581 A and U.S. Pat. No. 5,397,891. For each sensor grating there is a receiver grating covering the wavelength are of the sensor grating, where the reflected signal from the sensor grating and the receiver grating goes into a separate detector through a coupler. The detector signal is maximized using strain modulation of the receiver grating and feedback from the detector to the actuator adjusting the receiver grating. The actuator force will then be a measure for the Bragg wavelength of the sensor grating. This technique also implies significant optical losses in a wavelength multiplexed system as several fiberoptic couplers must be used. In addition one detector is required for each sensor.
Optical communication systems and optical networks uses in an increasing degree wavelength multiplexing for increasing the transmission capacity, which means that signals are transferred using a number of narrow banded semiconductor lasers with different optical wavelengths/frequencies separated typically by 50-200 GHz which may be transferred simultaneously along an optical fiber. In such systems simple, compact optical spectrum analysers may be used to measure important parameters such as laser wavelength, optical power and optical signal/noise ratio (spectral power in the centre of a laser line relative to a spectral power in a side band) in the different wavelength channels at a number of positions within the optical network.
The main object of this invention is to provide an inexpensive and practical device with a minimum of components, minimum optical loss and only one detector for accurate measuring of reflected Bragg wavelengths from one or more wavelength multiplexed fiber Bragg gratings being illuminated by a broad band optical source. It is also an object to this invention to provide an inexpensive and practical device with a minimum of components, minimal optical loss and only one detector for accurate measuring of laser wavelengths, optical power and optical signal/noise ration in the different optical channels of a wavelength multiplexed optical communication system.
The objects of the invention is obtained with a device with properties as specified in the characterizing part of claim 1. Additional properties are specified in the dependent claims.
The invention consists of using as a readout system for optical wavelengths, for example the Bragg wavelengths of one or more fiber Bragg gratings with different Bragg wavelengths, hereafter called sensor gratings, a pulsed broadband source, typically a light emitting diode, in a combination with an optical directional coupler and one I detector, and with separate analyses filters for each grating overlapping the grating spectrally, the analyses filters being based upon one or more fiber Bragg gratings. The analyses filters are positioned along an optical fiber with fiber delay lines (with fiber lengths in the range of 100 m) between each receiving grating, so that the reflected signals with different wavelengths for each sensor grating may be separated in time.
The analysis filter may consist of one Bragg grating which may be adjusted in wavelength using an actuator to lock on the same wavelength as the corresponding sensor grating. When the analysis filter and the sensor filter overlap the corresponding reflected pulse will have maximum power. The actuator force will in this case be a direct measure of the Bragg wavelength of the sensor grating. The actuator may be based on strain control or temperature in the analysis grating.
Alternatively the analysis filter may consist of to fibres being separated by a fiber delay line, where the two analysis gratings are partially overlapping spectrally and where the two Bragg wavelength of the sensor grating will be between the (flanks of) the two analysis gratings. The ratio between the reflected pulses from the two analysis gratings will thus be an indication of the spectral position of the sensor grating relative to the analysis gratings. The analysis gratings will have a constant temperature or a known, varying temperature. The analysis gratings may also be adjusted with an actuator based on controlled and known change of strain or temperature, to increase the measuring range.
The device according to the invention may also be used precise measuring of laser wavelengths, optical power and optical signal/noise ratio in the different wavelength channels in a wavelength multiplexed optical communication system, as laser light with different wavelengths from an optical communication fiber is retrieved through an optical coupler and sent through an optical switch, preferably with less than 1 xcexcs switching time, which pulses the light before it is sent through a directional coupler to a fiber with separate analysis filters for each wavelength channel overlapping spectrally the wavelength channel, the analysis filters being based on one or more Bragg gratings. The analysis gratings are positioned along one fiber with fiber delay lines, for example a the 100 m range, between each receiver grating, so that the reflected signals with different wavelengths for each wavelength channel may be separated in time.